Information gained from the determination of the condition of insulation of an electrical circuit is of principal importance. As system reliability is related to the condition of the insulation, insulation conditions can determine the reliability of the complete electrical system. The condition of electrical insulation can be determined by measuring different physical, electrical, and chemical characteristics. These characteristics are subject to change as a consequence of service stresses on the insulation. The measure of such changes allows conclusions to be drawn as to the condition and performance of the insulation.
The total form of insulation of a piece of electrical equipment or system may be called its insulating system. The deterioration of an insulating system can be of two different types:
1. A local deterioration or failure; and PA1 2. A general deterioration, spread over the bulk of the insulating system. PA1 a) moistening of the insulation; and PA1 b) accumulation of by-products developed by insulation deterioration at high temperature.
With respect to general deterioration of an insulation system, the detection of two types of changes to the system is of great importance. These changes are:
As between the two, moistening is especially significant. According to the results of research reported in IEE Proceedings, Vol. 132, Pt. 6 No. 6 (1986) p. 312-319, by D. H. Shroff and D. H.-A. W. Stannett, review of the aging of paper insulation in power transformers indicates that where a 2% humidity increase is experienced, the lifetime of such oil-paper insulating systems will be decreased to 1/20.sup.th of the dry insulation life.
Known prophylactic test processes for insulating systems are directed to determination of insulation resistance, the absorption factor, and the loss tangent of the system. The theoretical basis for such tests, and the first test apparatus, were developed more than 70 years ago. Although the design of testing apparatus has been brought up to date because of improvements in the implementing electronic circuitry, the information gained from such tests are of limited use.
The measurement of insulation resistance is one of the oldest insulation test methods. The advantage of this method is the simple procedure in carrying out the test, while its disadvantage is the rather low information content of the test results. The insulation resistance value is influenced not only by the aforementioned changes in the insulation, but also by numerous other parameters, among these the geometric dimensions of equipment or system under test. In addition, the relationship between insulation resistance and moisture and deterioration is not unambiguous; significant change in resistance is generally caused only be a great deterioration. Further, test results can be significantly influenced by the condition of the external insulation which affects the simplicity of the testing process as well.
The introduction of the absorption coefficient K.sub.A, a ratio of system resistance at two elapsed times after the application of a DC voltage, was promoted by two factors: 1) demand for a characteristic not depending on the geometric size of the test subject; and 2) experience which indicated that insulation resistance is dependent on the time of voltage application. Such a test typically uses a time ratio of 60 to 10, i.e. K.sub.A =R.sub.60 /R.sub.10, and the value of the measured insulation resistance is influenced by the polarization phenomena developed in the insulating system.
The evaluation of the absorption coefficient follows a rule of thumb: if the value of K.sub.A is between 2 and 2.5, then the condition of the insulating system can be regarded as good. If it is about 1 then the condition is regarded as bad. According to the investigations of the present inventors, the relationship between the absorption coefficient and the condition of the insulating system cannot be assessed so simply. This can be seen in FIG. 1, where K.sub.A =R.sub.60 /R.sub.10 is plotted against the humidity content of an oil-paper insulating system at different temperatures. The validity of the general assessment mentioned above can be strongly debated, as the absorption coefficient does not change monotonically as a function of humidity content. The disadvantage of even this improved method is that, because the superimposition of conduction and polarization phenomena is measured, its sensitivity is rather limited.
The determination of the loss factor or tangent (tan .delta.) has been used for more than 60 years for assessing the condition of electrical insulation in service. The value of the loss tangent is influenced by insulation losses of both the conduction and polarization character. The sensitivity of the loss tangent to certain polarization phenomena is primarily influenced by the frequency of the test voltage applied. Processes taking place during the aging of an insulation system have a strong influence on long time-constant polarization phenomena. As a result, tests carried out at a voltage of very low frequency (VLF) are the most sensitive, but practical on-site methods and apparatus for such VLF loss factor tests have not yet been developed. With loss tangent tests at industrial frequencies, significant resistance changes can be detected only in the case of a very greatly deteriorated insulating system. It is to be mentioned that on-site testing of loss tangent is a rather complicated testing process as strong electrical fields, present for example, in high voltage substations, have a disturbing effect on the test.
These traditional methods of insulation testing are of extremely limited usefulness because they characterize the condition of a perhaps very complicated insulating system by only one figure, which is of course not sufficient, and which is subject to qualification.
In spite of these facts, even with present testing techniques, only the above-mentioned methods are used for testing of electrical equipment in service. See, for example, the reported test results on 132 KV transformers in the United Kingdom in Domun-Cornfeld-Hadfield, Prediction of Remaining Lives of 132 kV Grid Transformers, CIGRE Symposium, Section 10.2, No 1020-08, (Vienna, May 1987).
The phenomena by which, after charging up by a direct current or voltage and subsequent discharging a return voltage appears, has been known for a long time as the electrical after-effect. See, for example, Andras Csernatony-Hoffer and Tibor Horvath, High Voltage Engineering(Tankonyvkiado, 1986) p. 260-65. In practical importance apparatus and equipment of high capacitance which are charged up by DC then switched off have to be practically short-circuited for a long time period, if not constantly. The electrical after-effect and the return voltage so developed contain information on the state of system insulation, as referred to in the literature along with its theoretical explanation.
The parameters of return voltage can give complete and full information on the state of insulation if the broad spectrum of polarization phenomena having long time-constants can be determined.
It is known that, with respect to certain values of charging and discharging times, a certain group of elementary polarization processes, characterized by their band of time-constants, can be activated. No test method and test apparatus were formerly known, however, for the broad implementation of such determination.
Thus, a purpose of the present invention is to provide a test methodology for the determination of polarization spectra in the domain of space charge polarization phenomena having long time-constants which would have an unambiguous relationship to the state and change of state of an insulating system by the evaluation of return voltage parameters appearing after charging the insulating system by a DC voltage and discharging it for a certain length of time.
A further purpose of the present invention is to provide a testing apparatus for implementing the test method.